Power supplies are employed to convert one voltage to another and may include converting from AC-DC, DC to AC, DC-DC or indeed AC-AC. DC-DC converters are devices which are employed to convert an input DC voltage to another DC voltage.
DC-DC converters may be described generally as either linear or switching converters. The present application is directed to both linear and switching converters. For convenience the background will now be described with reference to a switching DC-DC converter.
A conventional arrangement for a switching DC-DC converter 10 as shown in FIG. 1 uses a power stage 18 comprising one or more switching devices and one or more inductors and capacitors to convert an input voltage (Vin) to an output voltage (Vout). An analog compensator 14 is employed to try and maintain the output voltage at a desired set point, typically this is achieved by attempting to minimize the error corresponding to the difference 12 between the set point and output voltage. It will be appreciated that a gain may be applied to the output voltage in the feedback path to reduce this voltage for convenience in the circuitry but this is omitted to simplify the circuit for the purposes of explanation. Conventionally, pulse width modulation is employed to control the operation of the switching devices within the power stage 18, with the analog compensator 14 providing a control signal to a PWM module 16 which provides switching signals to one or more switching devices within the power stage 18. A variety of different switching circuit topologies may be employed within the power stage 18 which will be familiar to those skilled in the art, including for example the conventional buck and boost topologies, and switched capacitor types.
The analog compensator is typically designed to maximize the performance of the DC-DC converter in a typical circuit. Whilst this approach is suitable for most DC-DC applications, as user requirements increase it becomes difficult for conventional analog compensators to meet user requirements. There are several reasons for this including that the characteristics of the power stage may vary considerably from design values and as a result even a controller designed for a specific power stage and\or application may result in less than optimum control. In addition, other factors can affect the controller including the input voltage and load. It will be appreciated by those skilled in the art that a particular difficulty with control of DC-DC power supplies is the relatively high speed of switching of the switching elements of the power supply and the speed of corresponding load changes which can be nearly instantaneous in the case of electronic loads such as processors and similar logic circuitry.
It will be appreciated generally that adaptive control may be classified generally as either parametric or non-parametric. Online adaptive control is usually based upon a parametric model whereby the plant to be controlled is described by a model with various parameters. The values of the parameters are estimated online using parameter estimation and can therefore run continuously, without requiring a measurement phase or introducing disturbances into the system. These are usually recursive algorithms which estimate the values of the parameters in the chosen model. Examples of estimation algorithms include gradient estimation, least squares, recursive least squares with exponential forgetting, stochastic approximation. The estimated parameters may be derived directly or indirectly. Indirect parametric adaptive control methods require an estimator that outputs an estimate of the parameters of the model to the tuner. When direct parametric methods are employed tuning is based upon signals related to the parameters of the model and the estimation of the model parameters is therefore implicit.
Non-parametric adaptive control involves techniques such as transient analysis, frequency analysis, correlation analysis, in which certain properties of the system are identified, such as bandwidth, settling time etc. during a measurement phase. The requirement for a measurement phase disturbs closed-loop regulation. Non-parametric modelling is sensitive to noise making it hard to get accurate results and is not usually suitable for on-line system identification. Typically, non-parametric adaptive control methods can be conveniently described using flowcharts of the experiment phase; whereas parametric methods can run continuously and therefore a flowchart description is not generally applicable.
Saggini and Mattavelli in “A Simple Digital Auto-Tuning For Analog compensator in SMPS” discloses a non-parametric adaptive controller employing a tuning technique which introduces a non-linear gain into the control path during a tuning phase in which the controller parameters are tuned. The disadvantage of this approach is that practically it may only be used as part of an initial calibration and in common with non-parametric adaptive methods, is sensitive to disturbances. During use, the characteristics of the power stage may change and thus require further tuning. However, this would not be practical without disturbing the output voltage and would thus be undesirable.
As a result of the limitations inherent with analog compensators and so as to generally improve the performance of DC-DC controllers and provide greater functionality, it is known to implement the controller in digital form where more advanced control techniques may be employed. An example of a digital controller is described in U.S. Ser. No. 12/439,802, which is assigned to the present assignee, and the entire contents of which are hereby incorporated by reference. Whilst the technique proposed in this patent application offers significant advantages over the prior art, the use of digital controllers introduces a time delay which may limit the performance of the digital controller. Additionally, there is a familiarity in the market with analog compensators (albeit non-adaptive).
Whilst conceivably adaptive control may be implemented in an analog compensator, those skilled in the art will appreciate the problems including the requirement for a reference model response. Additionally, there are known problems generally associated with DC-offsets in analog multipliers which make these generally impractical.
Accordingly, there is a need for a controller for a power supply, such as for example a DC-DC converter, which addresses some or all of the problems associated with the prior art.